диафрагмированные волноводные фильтры / 8d1b2d95-355a-4a02-8126-4573c94a7aa0

Abstract— This paper provides consideration of a new technical solution of the band-pass filter (BPF). Microwave BPF consists of a cross waveguide with E-plane insert of hightemperature superconductivity (HTS) material. We have found that the insertion loss at contacts between HTS insert and a waveguide body is smaller than the loss in waveguide walls and HTS insert. This feature of the filter simplifies BPF manufacturing. Design procedure includes a calculation of the filter-prototype and a geometry synthesis by a modeling and electrodynamics computing. This filter has lower insertion loss than the filter with copper insert, and also increases reliability of design due to eliminating the causes of destruction of dielectric substrate with HTS material

Keywords— band-pass filter; HTS; cross waveguide

I.INTRODUCTION

Noise temperature of receivers is the main characteristic of high sensitive receivers. There are two things which determine the whole noise temperature of the receiver. The first is the noise temperature of low noise amplifier (LNA) and the second one is signal-noise ratio (SNR) in front of the LNA. Decreasing SNR in passive circuit include BPF at the input of LNA is equivalent increasing the noise temperature of the receiver. The smaller insertion loss of the BPF, the lower the noise temperature of the receivers. The BPF which made of materials with high conductivity or low value of the microwave surface resistance Rs, have advantage. The BPF designed with HTS materials have lower insertion loss then filters made from copper and silver under identical conditions. The value Rs of HTS materials by several orders is lower than Rs of normal metals but, these materials go over a superconducting state at a cryogenic cooling to the temperature of liquid nitrogen (about 77 K). This paper provides design of the BPF, which using HTS material in a form of HTS film deposited on the side surface of the dielectric plate (substrate) with low dielectric losses (e.g., superconducting layers of YBaCuO on MgO substrate).

For the first time the idea of using the inserts of HTS materials instead of fin-line inserts in the E-plane band-pass filters was mentioned in the work [1]. Experimental study of such a filter was carried out in the work [2]. Previously we found the characteristics of band-pass filter with E-plane insert of HTS material, and also compared the characteristics of band-pass filter with E-plane inserts of the normal metal [3].

The band-pass filter with E-plane insert of the HTS material cannot be realized with low insertion loss until the problem of contact between HTS insert and the waveguide walls is not solved. Contact area should have small losses of microwave power, ensure good thermal contact between the HTS insert and the waveguide walls and prevent the destruction of the fragile substrate plate in cooling-heating cycles of the filter.

II.PROPOSED HTS E-PLANE INSERT BPF DESIGN

Fig. 1 shows a perspective view of a proposed bandpass filter and Fig. 2 – a cross-section of the band-pass filter in the transverse plane of the waveguide [4]. Band-pass filter comprises a rectangular waveguide 1 of cross-section and a dielectric plate 2. Identical HTS films 3 with a definite number of windows 4 of the same height, which are symmetrically set in the both side of the plate relative to axes of a film. The windows 4 have different lengths and are located at different distances relative to each other. Specific dimensions are determined by the characteristics of band-pass filters, which should be designed. Plate 2 is mounted in the axial plane perpendicular to the wide walls of the waveguide 1. Dielectric plate 2 with films 3 of HTS material and rectangular windows 4 will be called hereinafter HTS insert. The length of rectangular grooves 5 equal to the length of the plate 2. The grooves are cut in both wide walls of the waveguide 1. The plate 2 is secured at the bottoms of the grooves 5. Fixing the plate 2 executes clamping between two identical half-bodies 6. Rectangular waveguide with the grooves in wide walls is often referred to cross waveguide in the literature [5].

Fig. 1 perspective view of the BPF

Fig. 2 transverse plane of the waveguide

For support of reliable thermal contact of HTS insert and half-bodies, it is desirable to interlay thin thermo-conducting layers 7. These layers can be for example, of indium foil, between conjugated surfaces of the insert and half-bodies. Per se in the invention, losses of microwave energy in layers 7 will be very small even at low electro conductivity of the layers. At the same time the layers 7 will eliminate mechanical stresses in a dielectric plate 2 during cooling-heating cycles and thereby preserve its destruction.

III.SYNTHESIS OF BPF

Electrodynamic synthesis of the proposed band-pass filter was carried out by means of «CST Microwave Studio». During the first stage, we synthesize the normalized filterprototype based on the required frequency characteristic of the microwave BPF. The synthesis is carried out using the general theory of synthesis of electrical circuits. This theory allows calculating a minimum number of elements for a predetermined characteristic of the microwave BPF. Synthesis prototype performs approximation of the frequency response by a polynomial of Chebyshev. Prototype contains the information necessary for the following stages of synthesis of microwave filter, i.e. the required number of cavities their resonance frequencies and the coefficients of mutual coupling, the values of external quality factors[6].

During the second stage we need to find initial approximations for (I) the resonator lengths (i.e. the window lengths), which determine eigen frequencies of the resonators, (II) the lengths of sections of mutual coupling (i.e. distance between the windows), which determine coefficients of mutual coupling of the resonators, and (III) the length of the end sections of coupling with input and output rectangular waveguides, which determine external Q-factor of the end resonators. For this purpose we use CST model of coupled resonators and calculate frequency dependencies of S- parameters. From these S parameters we reconstruct nominal values of the equivalent circuit, and calculate eigen frequencies and coefficients of mutual coupling of the paired resonators [7]. By changing the length of windows and distance between them, it is possible to achieve coincidence of reconstructed parameters of the equivalent circuit with parameters of the filter-prototype.

During the third stage the CST model of the pass-band filter is created as a number of elements which are found during the second stage. The lengths of resonators and distances between them are defined more accurately by means of optimization gradient method built-in «CST Microwave Studio» using the objective function, which is found on frequency response of the filter.

IV. RESULTS

The dependencies in Fig. 3 and Fig. 4 (see the curves 1 and 2 for material parameters of the filter elements without losses) show an example of synthesis of the eight-pole filter. Characteristics of this are: a central frequency is 30.5 GHz, a passband on –3 dB level is 1.2 GHz, a passband on –70 dB level is not more than 3 GHz, a return loss is less than -20 dB.

Fig.3 frequency response and return loss of the BPF

Curves 3 and 4 in Fig. 3 and Fig. 4 show the frequency characteristics of the synthesized filter taking into account Joule loss in the filter elements cooled to 77K. When calculating, loss tangent of the dielectric MgO substrate tanδ = 6.210-6, conductivity of the metal walls of the

waveguide σ Ag = 5.56 108 S/m and the equivalent conductivity

of the HTS material σ HTS = 1.0 1010 S/m were set. We see that the expected insertion loss of the eight-pole filter does not exceed 0.2 dB. Similar calculated characteristics can be obtained for band-pass filter with E-plane HTS insert in a rectangular waveguide, assuming no loss in the area of contact of HTS insert with the waveguide body. In practice, the losses are always present at the contacts.

Fig.4 frequency response of the BPF with losses (2) and without losses (4)

The rectangular grooves in a wide rectangular waveguide walls (cross waveguide) reduce losses in the HTS insert

contacts with the waveguide to a negligible value. A quantitative estimate of this effect was carried out. Fig. 5 shows the dependence on the Joule losses in the various depth of the grooves. Obviously, in the special case for d = 0, a cross waveguide is transformed into a rectangular waveguide. Losses were calculated in the horizontal and vertical walls of the waveguide (curves 1 and 2), in the HTS layers (curve 4), a dielectric plate (curve 5) and finally in the contacts (curve 3) simulated the metal layers with low conductivity (σ C =1.0 105

S/m) of 0.05 mm thickness. Relative losses in these filter elements are shown when taken as 100% total loss for each case of the geometry, i.e. for particular groove depth. For design as whole absolute losses (curve 6) were calculated in all cases of the filter geometry for 1 W incident power.

We observe that with increasing depth of the grooves, total losses initially decrease fast in absolute value and, since certain value (in this case d ≈ 0.5 mm) remain approximately constant. This happens due to lower losses in the contacts. For d > 0.5 mm, the loss of contact is comparable with losses of other components.

Fig.5 Relative losses in the elements of the filter

Fig. 6 Comparing insertion loss of BPF with different materials

As known, the insertion losses in the pass-band are determined by the relationship between its eigen Q0 and the

external QEX Q-factors of the filter resonator. External Q-

factor determines the bandwidth. When reducing the bandwidth it is necessary to increase the external quality factor. This increases ratio Qex / Q0 and increases the insertion loss. The

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advantage using HTS inserts will be appeared for narrow-band filters. Fig. 6 shows the calculated insertion loss for two-pole Ka band filters with a bandwidth of 250 MHz with both E- plane copper insert and HTS one. The calculation is performed for the electro physical parameters of materials at 77K. As we see, in the case of a two-pole filter the insertion loss of the filter with HTS insert is L =0.06 dB it compared with the filter with metal insert. With the growing number of the filter poles, insert losses increase and therefore the feasibility of HTS inserts increases. For the eight-pole filter one can obtain

L =0.2 dB, which is already quite significant value.

V.CONCLUSION

The use of the BPF with E-plane HTS insert is rational in cryo-electronic units of the microwave high-sensitive receivers where require using narrow-band filters with steep fronts. The proposed technical solution of the BPF with E-plane HTS insert in comparison with the known solutions has advantage in the insertion loss due to a reduction of loss in contact area between HTS insert and a waveguide body. In this solution increases reliability of design due to eliminating the causes of destruction of dielectric substrate with HTS material.

REFERENCES

[1]Mansour R.R., Zybura A. Superconducting Millimeter - Wave E - Plane Filters // IEEE Trans. Microwave Theory Tech. Vol. 39, No. 9, 1991, pp.1588-1492.

[2]Liang Han, Yiyuan Chen, Yunyi Wang. Design and Performance of WaveguidevЕ - Plane HTSC Insert Filters / 1992 IEEE MTT - S Digest, рр. 913-916.

[3]Skresanov V.N., Barannik A.A., Cherpak N.T., Y. He, Glamazdin V.V., Zolotarev V.A., Shubny A.I., Sun L., Wang J., Wu Y. / Experience in Developing Ka Band Waveguide Filter with HTS E Plane Insert / MSMW’2013, Kharkov, Ukraine, June 23 28, 2013.

[4]Band-Pass Filter / V.N. Skresanov, A.A. Barannik, V.V. Glamazdin, V.A. Zolotarev, M.P. Natarov N.T. Cherpak, A.I. Shubny, Yusheng

He, Liang Sun, Jia Wang, Xu Wang, Yun Wu3 / Favorable decision for Patent № a 2013 15299, Ukraine, 19th may 2015.

[5]Tham Q.C. Modes and Cutoff Frequencies of Crossed Rectangular Waveguides / IEEE Trans. Microwave Theory Tech. Vol. 25, No. 7,1977, pp. 585-588.

[6]J.L. Matthae, L. Young, E.M.T. Jones, Microwave Filters, Impedance-Matching Networks, and Coupling Structures - McGrawHill Co., 1968.

[7]V.N. Skresanov, V.V. Glamazdin, N.T. Cherpak "The Novel Approach to Coupled Mode Parameters Recovery from Microwave Resonator Amplitude - Frequency Response", European Microwave Conference. (EuMW 2011 Conference Proceedings). 9-14 October 2011. - Manchester, UK. - EuMA. - 2011. - pp. 826-829.

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